Characterization of synthetic filaments prepared from column-purified rabbit skeletal myosin indicates that, within certain limitations, these aggregates can be used as a model system for the study of thick filament protein interactions. This system is currently being applied in several related areas. C-protein has been shown to affect synthetic filament structure within a certain narrow range of proportion to myosin, and the nature of its effect is being investigated through studies of the aggregation process or final filament structure are being studied. These studies will be extended to other thick filament proteins in the near future. The structure of the thick filament of skeletal muscle remains a controversial and unanswered question. We are using high magnification electron microscopy and various image analysis techniques to obtain increased information about natural filament characteristics. It is hoped that these studies, as well as the synthetic filament work mentioned above, will lead to an increased understanding of the natural thick filament and the function of its component proteins.